This invention relates to a modulation technology used in digital modulation communication system using a plurality of multi-carriers and, more particularly to the art of controlling an envelope of a transmission modulated wave.
A conventional power amplifier connected to a rear step of a multi-carrier digital modulator has a saturation level. With the above power amplifier, it is well-known that excessive increase in the input level may widen spectrum of the output signal due to clipping distortion.
With a communication system in which a plurality of multi-carriers are digitally modulated with transmission data for transmission, a peak power value is likely to be excessively great to an average power of transmission signals. Such system needs to operate the power amplifier at a working point at sufficiently lower than its saturation level.
This may cause the output power to be decreased simultaneously, requiring to enlarge the power amplifier for obtaining required output power.
The Japanese Patent Laid-Open No.13156(1990) has disclosed the art to solve the aforementioned problem. The disclosed radio communication unit is described referring to FIG. 4.
Two transmission data (DATA1 and DATA2) synchronized with a clock signal CLK are so set to locate each data variation point shifted by a half clock cycle in a timing circuit 801, which are output as X1 and X2, respectively.
The outputs X1 and X2 of the timing circuit 801 are subjected to band-limit through low pass filters 802 and 803 to become Y1 and Y2, respectively. The Y1 and Y2 output from the low pass filters 802 and 803 are input to PSK modulators 804 and 805, respectively. The PSK modulators 804 and 805 serve to produce two PSK waves through phase shift-keying modulation of carriers from carrier generators 806 and 807 with the input signals Y1 and Y2, respectively.
The two PSK waves are synthesized in a synthesizer 808. The band-limited PSK modulated waves produced by the PSK modulators 804 and 805 have the respective data variation points shifted by a half clock cycle. That is, each maximum amplitude position of the two PSK modulated waves is located so as not to interlock with each other.
As a result, the disclosed communication unit allows the maximum amplitude output from the synthesizer 808 to be smoothed to output signals at almost uniform amplitude level.
Assuming that the multi-carrier is designated as "n waves" in the above prior art, it may allow each data variation point to be shifted by 1/n of the clock cycle.
With this prior art, in order to synthesize a plurality of multi-carriers modulated through digital modulation system, for example, 2-value digital modulation system as a PSK modulation system, which causes a modulated wave to always get through the origin (amplitude:0) on IQ plane when it is between one signal point and another, each modulated wave is expected to be located so as to prevent interlocking of maximum amplitude positions. The prior art is effective for obtaining signals at almost uniform amplitude level.
While using other modulation system, for example, 8-phase PSK, .pi./4 shift QPSK, and 16QAM, as multi values (three values or more) digital modulation system, the modulated wave does not always get through the origin even when it is between one signal point and another.
Although each variation point of the data is shifted by 1/n of the clock cycle, the modulated wave of n wave may be located at the position where each maximum amplitude is interlocking. Accordingly the effect of "smoothing the maximum amplitude of the output to cause the signal at almost uniform amplitude level" cannot be always anticipated. This prior art, therefore, has failed to improve power efficiency of the power amplifier connected to the rear step.